Tone signalling has been commonly used in communication systems to provide an address so that only a particular station responds to a call. Such systems have included decoders utilizing tone filter elements to respond to the desired tone frequencies. These filter elements have been resonant reed devices, active filters or other expensive elements, and it has been necessary to provide a separate element for each tone used so that the decoders have been relatively costly. In order to change the frequency or frequencies to which such decoders respond, it has been necessary to change the filter element, thus requiring a relatively major change in the decoder. Also, these decoders have been suitable for use with only one tone code form or sequence, and cannot be easily modified to operate with a different code.
Known tone detectors have not provided the sensitivity and selectivity characteristics required for certain applications. Reed devices which may have high selectivity are, in addition to being expensive, quite large and therefore not suitable for use in miniature apparatus. Further, known tone decoder systems have not been adaptable to implementation in the form of a single large scale integrated circuit chip. This has made the decoder system objectionably expensive and not suitable for use in some tone signalling systems.